Membrane fusion mediated by influenza virus hemagglutinin is among the best characterized fusion reactions. However a little is known about the fusion stages between low-pH-dependent change in HA conformation, which triggers fusion, and low-pH-independent actual mixing of the lipids and aqueous contents of the membranes. By altering the number of activated Has, temperature, and membrane lipid composition, the low-pH-independent processes in the fusion of HA-expressing cells with human red blood cells were dissected into distinct stages. Low temperature (<60 C) reversibly arrested the stage of local membrane merger, which is upstream of both lipid mixing and fusion pore formation. While low pH conformations of HA were required to achieve and to stabilize this fusion intermediate, and to complete fusion upon raising temperature, the very same proteins restricted lipid mixing. Local merger of membranes to form this intermediate was dependent on the lipid composition of the outer membrane monolayers. Similar intermediates were found not only for complete fusion, but also for long-living hemifusion state (lipid mixing without content mixing) observed at lowered numbers of the HAs, and for mutant, glycosylphosphatidylinositol-anchored HA. For wild type HAs, switching any given cell-cell contact towards pore development and away from stable hemifusion depends on the number of activated HA and their mobility, as well as on the lipid composition of the inner monolayers of fusing membranes which would assemble in forming a hemifusion diaphragm. Our data are consistent with the hypothesis that after the conformational changes in HA are complete, trimers must assemble to form a fusion complex which supports first a local lipidic connection of fusing membranes. Rather than dissipating into extended hemifusion, this restricted hemifusion is the site of lipidic fusion pore formation, which then expands by dilution of HA.